<!DOCTYPE html>
<html>

<head>
  <meta charset="utf-8">
  <link rel="stylesheet" href="style.css">
  <script src="https://cdn.polyfill.io/v1/polyfill.js?features=Element.prototype.closest"></script>
  <script src="voter.js"></script>
  <script src="step-voter.js"></script>
  <!-- отнаследовать и переопределить методы -->
</head>

<body>

  <div id="voter" class="voter">
    <span class="down">–</span>
    <span class="vote">0</span>
    <span class="up">+</span>
  </div>

  <script>
    var voter = new StepVoter({
      elem: document.getElementById('voter'),
      step: 2
    });
  </script>

</body>

</html>